Prevent flush on closed NamedStream (#3507)

* Prevent flush on closed NamedStream

* update changelog and docs

* revert versionchanged entry

package/CHANGELOG

@@ -19,6 +19,7 @@ The rules for this file:
  * 2.1.0
 
 Fixes
+  * Prevents attempts to close an already closed NamedStream (Issue #3386)
   * Added missing exe_err import in hole.py. Fixed a bug in HoleAnalysis where 
     kwarg was passed instead of object to generate path (Issue #3493, PR #3496)
   * Fixes creation of vmd surfaces in hole2 when using non-contiguous frames

package/MDAnalysis/lib/util.py

@@ -725,6 +725,9 @@ def close(self, force=False):
                   always closes the underlying stream.
 
         """
+        if self.closed:
+            return
+
         if self.close_stream or force:
             try:
                 return self.stream.close()

testsuite/MDAnalysisTests/utils/test_streamio.py

@@ -96,6 +96,8 @@ def test_closing_force(self):
         assert_equal(ns.closed, False)
         ns.close(force=True)
         assert_equal(ns.closed, True)
+        # Issue 3386 - calling close again shouldn't raise an error
+        ns.close()
 
     def test_StringIO_read(self):
         obj = StringIO("".join(self.text))